docs(sftp): document server operations (#3391)

Adds an operator guide for the SFTP server: recommended
configuration, the path model, host keys on Unix and Windows, the
environment variable reference, session cleanup behaviour, IAM
policy requirements per SFTP operation, client compatibility
notes, multipart upload sizing and cleanup, and the log lines
worth alerting on.

Corrects documentation the platform change left stale. The module
overview and the UnsupportedPlatform error text still described
Windows as unsupported. A source comment referenced a document
that does not exist in the repository and now points at the new
guide. The changelog adds the two host-key reload variables
missing from its environment list, describes the banner variable
as the SSH identification string, and corrects the upload size
cap and compliance case count.

crates/protocols/src/sftp/config.rs

@@ -85,12 +85,12 @@ pub enum SftpInitError {
     #[error("SSH server error: {0}")]
     Server(String),
 
-    /// The host running the binary is not a Unix-family target. The
-    /// host-key permission enforcement (the owner-only mode-bit check)
-    /// requires Unix mode bits and has no equivalent on this platform,
-    /// so SFTP refuses to start rather than load host keys with weaker
-    /// guarantees.
-    #[error("SFTP requires a Unix-family host (current OS: {os})")]
+    /// The host running the binary is neither a Unix-family target nor
+    /// Windows. Unix enforces owner-only host-key mode bits and Windows
+    /// trusts operator-managed NTFS ACLs, but neither mechanism exists on
+    /// other targets, so SFTP refuses to start rather than load host keys
+    /// with weaker guarantees.
+    #[error("SFTP requires a Unix-family or Windows host (current OS: {os})")]
     UnsupportedPlatform { os: String },
 }